1.1. Overview

This module implements the TLS transport for SIP-router using the OpenSSL library (http://www.openssl.org). To enable the TLS support this module must be loaded and enable_tls=yes must be added to the SIP-router config file

1.2. Quick Start

Make sure you have a proper certificate and private key and either use the certificate and private_key module parameters, or make sure the certificate and key are in the same PEM file, named cert.pem an placed in [your-cfg-install-prefix]/etc/ser/. Don't forget to load the tls module and to enable TLS (add enable_tls=yes to your config).

1.3. Important Notes

The TLS module needs some special options enabled when compiling SIP-router. These options are enabled by default, however in case you're using a modified SIP-router version or Makefile, make sure that you enable -DUSE_TLS and -DTLS_HOOKS (or compile with make TLS_HOOKS=1 which will take care of both options). To quickly check if your SIP-router version was compiled with these options, run ser -V and look for USE_TLS and TLS_HOOKS among the flags.

This module includes several workarounds for various Openssl bugs (like compression and Kerberos using
the wrong memory allocations functions, low memory problems a.s.o). On startup it will try to enable
the needed workarounds based on the openssl library version. Each time a known problem is detected and
a workaround is enabled, a message will be logged. In general it is recommended to compile this module on the same machine or a similar machine to where ser will be run or to link it statically with libssl. For example if on the compile machine openssl does not have the kerberos support enabled, but on the target machine a kerberos enabled openssl library is installed, ser cannot apply the needed workarounds and will refuse to start. The same thing will happen if the openssl versions are too different (to force ser startup anyway, see the tls_force_run module parameter).

Compression is fully supported if you have a new enough Openssl version (starting with 0.9.8). Although there are some problems with zlib compression in currently deployed Openssl versions (up to and including 0.9.8d, see openssl bug #1468), the TLS module will automatically switch to its own fixed version.
Note however that starting with sr 3.1 compression is not enabled by default, due to the huge extra memory consumption that it causes (about 10x more memory). To enable it use modparam("tls", "tls_disable_compression", 0)
(see tls_disable_compression).

The TLS module includes workarounds for the following known openssl bugs: openssl #1204 (disable SS_OP_TLS_BLOCK_PADDING_BUG if compression is enabled, for versions between 0.9.8 and 0.9.8c), openssl #1468 (fix zlib compression memory allocation), openssl #1467 (kerberos support will be disabled if the openssl version is less than 0.9.8e-beta1) and openssl #1491 (stop using tls in low memory situations due to the very high risk of openssl crashing or leaking memory). The bug reports can be viewed at
http://rt.openssl.org/.

1.4. Compiling the TLS Module

In most case compiling the TLS module is as simple as:

make -C modules/tls

or

make modules modules=modules/tls

or (compiling whole SIP-router and the tls module)

make all include_modules=tls

.

However in some cases the openssl library requires linking with other libraries. For example compiling the openssl library with kerberos and zlib-shared support will require linking the tls module with libkrb5 and libz. In this case just add TLS_EXTRA_LIBS="library list" to make's command line. E.g.:

make TLS_EXTRA_LIBS="-lkrb5 -lz" all include_modules=tls

In general, if SIP-router fails to start with a symbol not found error when trying to load the tls module (check the log), it means some needed library was not linked and it must be added to TLS_EXTRA_LIBS

1.5. TLS and Low Memory

The openssl library doesn't handle very well low memory situations. If memory allocations start to fail (due to memory shortage), openssl can crash or cause memory leaks (making the memory shortage even worse). As of this writing all openssl versions were affected (includind 0.9.8e), see openssl bug #1491. The tls module has some workarounds for preventing this problem (see low_mem_treshold1 and low_mem_threshold2), however starting SIP-router with enough shared memory is higly recommended. When this is not possible a quick way to significantly reduce openssl memory usage it to disable compression (see tls_disable_compression).

1.6. TLS Debugging

Debugging messages can be selectively enabled by recompiling
the tls module with a combination of the following defines:

TLS_WR_DEBUG - debug messages for the write/send part.

TLS_RD_DEBUG - debug messages for the read/receive
part.

TLS_BIO_DEBUG - debug messages for the custom BIO.

Example 2. Compiling TLS with Debug Messages

make -C modules/tls extra_defs="-DTLS_WR_DEBUG -DTLS_RD_DEBUG"

To change the level at which the debug messages are logged,
change the tls_debug module parameter.

1.7. Known Limitations

The private key must not encrypted (SIP-router cannot ask you for a password on startup).

The TLS certificate verifications ignores the certificate name, subject altname and ip extensions, it just checks if the certificate is signed by a recognized CA. One can use the select framework to try to overcome this limitation (check in the script for the contents of various certificate fields), but this is not only slow, but also not exactly standard conforming (the verification should happen during TLS connection establishment and not after).

TLS specific config reloading is not safe, so for now better don't use it, especially under heavy traffic.

1.8. Quick Certificate Howto

There are various ways to create, sign certificates and manage small CAs (Certificate Authorities). If you want a GUI, try tinyca (http://tinyca.sm-zone.net/), a very nice and small CA management application. If you are in a hurry and everything you have are the installed openssl libraries and utilities, read on.

Assumptions: we run our own CA.

Warning: in this example no key is encrypted. The client and server private keys must not be encrypted (ser doesn't support encrypted keys), so make sure the corresponding files are readable only by trusted people. You should use a password for your CA private key.

1.9. Parameters

1.9.1. tls_method (string)

TLSv1 - only TLSv1 connections are accepted. This is the default and recommended method (if you want to be rfc3261 conformant don't change it).

SSLv3 - only SSLv3 connections are accepted

SSLv2 - only SSLv2 connections, for old clients. Note: you shouldn't use SSLv2 for anything which should be highly secure.

SSLv23 - any of the above methods will be accepted, with the following limitation: the initial SSL hello message must be V2 (in the initial hello all the supported protocols are advertised enabling switching to a higher and more secure version). This means connections from SSLv3 or TLSv1 clients will not be accepted.

If rfc3261 conformance is desired, TLSv1 must be used. For compatibility with older clients SSLv23 is a good option.

Example 3. Set tls_method parameter

...
modparam("tls", "tls_method", "TLSv1")
...

1.9.2. certificate (string)

Sets the certificate file name. The certificate file can also contain
the private key in PEM format.

If the file name starts with a '.' the path will be relative to the
working directory (at runtime). If it starts
with a '/' it will be an absolute path and if it starts with anything
else the path will be relative to the main config file directory
(e.g.: for ser -f /etc/ser/ser.cfg it will be relative to /etc/ser/).

Warning: try not to use certificate with keys
longer then 1024 bytes. Longer keys will severely impact performance,
in particular the TLS connection rate.

1.9.3. private_key (string)

Sets the private key file name.

If the file name starts with a '.' the path will be relative to the
working directory (at runtime). If it starts
with a '/' it will be an absolute path and if it starts with anything
else the path will be relative to the main config file directory
(e.g.: for ser -f /etc/ser/ser.cfg it will be relative to /etc/ser/).

Note: the private key can be contained in the same file as the
certificate (just append it to the certificate file, e.g.:
cat pkey.pem >> cert.pem)

The default value is [SER_CFG_DIR]/cert.pem.

Example 5. Set private_key parameter

...
modparam("tls", "private", "/usr/local/etc/ser/my_pkey.pem")
...

1.9.4. ca_list (string)

Sets the CA list file name. This file contains a list of all the
trusted CAs certificates. If a signature in a certificate chain belongs
to one of the listed CAs, the authentication will succeed.

If the file name starts with a '.' the path will be relative to the
working directory (at runtime). If it starts
with a '/' it will be an absolute path and if it starts with anything
else the path will be relative to the main config file directory
(e.g.: for ser -f /etc/ser/ser.cfg it will be relative to /etc/ser/).

By default the CA file is not set.

An easy way to create the CA list is to append each trusted trusted CA
certificate in the PEM format to one file, e.g.: for f in
trusted_cas/*.pem ; do cat "$f" >> ca_list.pem ; done .

See also
verify_certificate,
verify_depth,
require_certificate and
crl.

Example 6. Set ca_list parameter

...
modparam("tls", "ca_list", "/usr/local/etc/ser/ca_list.pem")
...

1.9.5. crl (string)

Sets the certificate revocation list file name. This file contains a
list of revoked certificates. Any attempt to verify a revoked
certificate will fail.

If not set, no crl list will be used.

If the file name starts with a '.' the path will be relative to the
working directory (at runtime). If it starts
with a '/' it will be an absolute path and if it starts with anything
else the path will be relative to the main config file directory
(e.g.: for ser -f /etc/ser/ser.cfg it will be relative to /etc/ser/).

Note

If set, require_certificate should also be set
or it will not have any effect.

By default the crl file is not set.

To update the crl in a running ser, make sure you configure tls
via a separate tls config file
(the config modparam) and issue a tls.reload
RPC call, e.g.:

See also
ca_list,
verify_certificate,
verify_depth and
require_certificate.

Example 7. Set crl parameter

...
modparam("tls", "crl", "/usr/local/etc/ser/crl.pem")
...

1.9.6. verify_certificate (boolean)

If enabled it will force certificate verification. For more information see the verify(1) openssl man page.

Note: the certificate verification will always fail if the ca_list is empty.

See also: ca_list, require_certificate, verify_depth.

By default the certificate verification is off.

Example 8. Set verify_certificate parameter

...
modparam("tls", "verify_certificate", 1)
...

1.9.7. verify_depth (integer)

Sets how far up the certificate chain will the certificate verification go in the search for a trusted CA.

See also: ca_list, require_certificate, verify_certificate,

The default value is 9.

Example 9. Set verify_depth parameter

...
modparam("tls", "verify_depth", 9)
...

1.9.8. require_certificate (boolean)

When enabled it will require a certificate from a client. If the client does not offer a certificate and verify_certificate is on, the certificate verification will fail.

The default value is off.

Example 10. Set require_certificate parameter

...
modparam("tls", "require_certificate", 1)
...

1.9.9. cipher_list (string)

Sets the list of accepted ciphers. The list consists of cipher strings separated by colons. For more information on the cipher list format see the cipher(1) openssl man page.

The default value is not set (all the Openssl supported ciphers are enabled).

Example 11. Set cipher_list parameter

...
modparam("tls", "cipher_list", "HIGH")
...

1.9.10. send_timeout (int)

This parameter is obsolete and cannot be used
in newer TLS versions (> sip-router 3.0). In these versions the
send_timeout is replaced by tcp_send_timeout
(common with all the tcp connections).

1.9.11. handshake_timeout (int)

This parameter is obsolete and cannot be used
in newer TLS versions (> sip-router 3.0). In these versions the
handshake_timeout is replaced by tcp_connect_timeout
(common with all the tcp connections).

1.9.12. connection_timeout (int)

Sets the amount of time after which an idle TLS connection will be
closed, if no I/O ever occured after the initial open. If an I/O event
occurs, the timeout will be extended with tcp_connection_lifetime.
The value is expressed in seconds.

The default value is 10 min.

If the value set is -1, the connection will never be close on idle.

It can be changed also at runtime, via the RPC interface and config
framework. The config variable name is tls.connection_timeout.

Example 12. Set connection_timeout parameter

...
modparam("tls", "connection_timeout", 60)
...

Example 13. Set tls.connection_timeout at runtime

$ sercmd cfg.set_now_int tls connection_timeout 180

1.9.13. tls_disable_compression (boolean)

If set compression over SSL/TLS will be disabled.
Note that compression uses a lot of memory (about 10x more then with
the compression disabled), so if you want to minimize
memory usage is a good idea to disable it.

By default compression is disabled.

Example 14. Set tls_disable_compression parameter

...
modparam("tls", "tls_disable_compression", 0) # enable
...

1.9.14. ssl_release_buffers (integer)

Release internal OpenSSL read or write buffers as soon as they are
no longer needed. Combined with
ssl_free_list_max_len has the potential of saving
a lot of memory ( ~ 32k per connection in the default configuration,
or 16k + ssl_max_send_fragment).
For sr versions > 3.0 it makes little sense to disable it (0)
since the tls module already has its own internal buffering.

A value of -1 would not change this option from its openssl default.
Use 0 or 1 for enable/disable.

By default the value is 1 (enabled).

Note

This option is supported only for
OpenSSL versions >= 1.0.0.
On all the other versions attempting
to change the default will trigger an error.

Example 15. Set ssl_release_buffers parameter

modparam("tls", "ssl_release_buffers", 1)

1.9.15. ssl_free_list_max_len (integer)

Sets the maximum number of free memory chunks, that OpenSSL will keep
per connection. Setting it to 0 would cause any unused memory chunk
to be immediately freed, reducing the memory footprint. A too large
value would result in extra memory consumption.

Should be combined with ssl_release_buffers.

A value of -1 has a special meaning: the OpenSSL default will be used
(no attempt on changing the value will be made). For OpenSSL 1.0
the internal default is 32.

By default the value is 0 (no freelist).

Note

This option is supported only for
OpenSSL versions >= 1.0.0.
On all the other versions attempting
to change the default will trigger an error.

Example 16. Set ssl_freelist_max_len parameter

modparam("tls", "ssl_freelist_max_len", 0)

1.9.16. ssl_max_send_fragment (integer)

Sets the maximum number of bytes (from the clear text) sent into
one TLS or SSL record. Valid values are between 512 and 16384.
Note however that even valid low values might not be big enough to
allow a succesfull handshake (try minimum 1024).

Lower values would lead to less memory usage, but values lower then
the typical ser/sip-router write size would incur a slight performance
penalty. Good values are bigger then the size of the biggest
SIP packet one normally expects to forward. For example in most
setups 2048 would be a good value.

Note

Values on the lower side, even if valid (> 512), might not allow
for a succesfull initial handshake. This happens if the
certificate does not fit inside one send fragment.
Values lower then 1024 should not be used.
Even with higher values, if the handshake fails,
try increasing the value.

A value of -1 has a special meaning: the OpenSSL default will be used
(no attempt on changing the value will be made).

By default the value is -1 (the OpenSSL default, which at least in
OpenSSL 1.0.0 is ~ 16k).

Note

This option is supported only for
OpenSSL versions >= 0.9.9.
On all the other versions attempting
to change the default will trigger an error.

Example 17. Set ssl_max_send_fragment parameter

modparam("tls", "ssl_max_send_fragment", 4096)

1.9.17. ssl_read_ahead (boolean)

Enables read ahead, reducing the number of internal OpenSSL BIO read()
calls. This option has only debugging value, in normal circumstances
it should not be changed from the default.

When disabled OpenSSL will make at least 2 BIO read() calls per
received record: one to get the record header and one to get the
rest of the record.

The TLS module buffers internally all read()s and defines its own fast
BIO so enabling this option would only cause more memory consumption
and a minor slow-down (extra memcpy).

A value of -1 has a special meaning: the OpenSSL default will be used
(no attempt on changing the value will be made).

By default the value is 0 (disabled).

Example 18. Set ssl_read_ahead parameter

modparam("tls", "ssl_read_ahead", 1)

1.9.18. send_close_notify (boolean)

Enables/disables sending close notify alerts prior to closing the
corresponding TCP connection. Sending the close notify prior to tcp
shutdown is "nicer" from a TLS point of view, but it has a measurable
performance impact. Default: off. Can be set at runtime
(tls.send_close_notify).

The default value is 0 (off).

It can be changed also at runtime, via the RPC interface and config
framework. The config variable name is tls.send_close_notify.

Example 19. Set send_close_notify parameter

...
modparam("tls", "send_close_notify", 1)
...

Example 20. Set tls.send_close_notify at runtime

$ sercmd cfg.set_now_int tls send_close_notify 1

1.9.19. con_ct_wq_max (integer)

Sets the maximum allowed per connection clear-text send queue size in
bytes. This queue is used when data cannot be encrypted and sent
immediately because of an ongoing TLS/SSL level renegotiation.

The default value is 65536 (64 Kb).

It can be changed also at runtime, via the RPC interface and config
framework. The config variable name is tls.con_ct_wq_max.

Example 21. Set con_ct_wq_max parameter

...
modparam("tls", "con_ct_wq_max", 1048576)
...

Example 22. Set tls.con_ct_wq_max at runtime

$ sercmd cfg.set_now_int tls con_ct_wq_max 1048576

1.9.20. ct_wq_max (integer)

Sets the maximum total number of bytes queued in all the clear-text
send queues. These queues are used when data cannot be encrypted and
sent immediately because of an ongoing TLS/SSL level renegotiation.

The default value is 10485760 (10 Mb).

It can be changed also at runtime, via the RPC interface and config
framework. The config variable name is tls.ct_wq_max.

1.9.21. ct_wq_blk_size (integer)

It can be changed also at runtime, via the RPC interface and config
framework. The config variable name is tls.ct_wq_blk_size.

Example 25. Set ct_wq_blk_size parameter

...
modparam("tls", "ct_wq_blk_size", 2048)
...

Example 26. Set tls.ct_wq_max at runtime

$ sercmd cfg.set_now_int tls ct_wq_blk_size 2048

1.9.22. tls_log (int)

Sets the log level at which TLS related messages will be logged.

The default value is 3 (L_DBG).

It can be changed also at runtime, via the RPC interface and config
framework. The config variable name is tls.log.

Example 27. Set tls_log parameter

...
# ignore TLS messages if SIP-router is started with debug less than 10
modparam("tls", "tls_log", 10)
...

Example 28. Set tls.log at runtime

$ sercmd cfg.set_now_int tls log 10

1.9.23. tls_debug (int)

Sets the log level at which TLS debug messages will be logged.
Note that TLS debug messages are enabled only if the TLS module
is compiled with debugging enabled (e.g. -DTLS_WR_DEBUG,
-DTLS_RD_DEBUG or -DTLS_BIO_DEBUG).

The default value is 3 (L_DBG).

It can be changed also at runtime, via the RPC interface and config
framework. The config variable name is tls.debug.

Example 29. Set tls_debug parameter

...
# ignore TLS debug messages if SIP-router is started with debug less than 10
modparam("tls", "tls_debug", 10)
...

Example 30. Set tls.debug at runtime

$ sercmd cfg.set_now_int tls debug 10

1.9.24. low_mem_threshold1 (integer)

Sets the minimal free memory from which attempts to open or accept
new TLS connections will start to fail. The value is expressed in KB.

The default value depends on whether the openssl library used handles well low memory situations (openssl bug #1491). As of this writing this is not true for any openssl version (including 0.9.8e).

If an ill-behaved openssl version is detected, a very conservative value is choosed, which depends on the maximum possible number of simultaneously created TLS connections (and hence on the process number).

The following values have a special meaning:

-1 - use the default value

0 - disable (TLS connections will not fail preemptively)

It can be changed also at runtime, via the RPC interface and config
framework. The config variable name is tls.low_mem_threshold1.

See also low_mem_threshold2.

Example 31. Set low_mem_threshold1 parameter

...
modparam("tls", "low_mem_threshold1", -1)
...

Example 32. Set tls.low_mem_threshold1 at runtime

$ sercmd cfg.set_now_int tls low_mem_threshold1 2048

1.9.25. low_mem_threshold2 (integer)

Sets the minimal free memory from which TLS operations on already established TLS connections will start to fail preemptively. The value is expressed in KB.

The default value depends on whether the openssl library used handles well low memory situations (openssl bug #1491). As of this writing this is not true for any openssl version (including 0.9.8e).

If an ill-behaved openssl version is detected, a very conservative value is choosed, which depends on the maximum possible number of simultaneously created TLS connections (and hence on the process number).

The following values have a special meaning:

-1 - use the default value

0 - disable (TLS operations will not fail preemptively)

It can be changed also at runtime, via the RPC interface and config
framework. The config variable name is tls.low_mem_threshold2.

See also low_mem_threshold1.

Example 33. Set low_mem_threshold2 parameter

...
modparam("tls", "low_mem_threshold2", -1)
...

Example 34. Set tls.low_mem_threshold2 at runtime

$ sercmd cfg.set_now_int tls low_mem_threshold2 1024

1.9.26. tls_force_run (boolean)

If enabled SIP-router will start even if some of the openssl sanity checks fail (turn it on at your own risk).

Currently failing any of the following sanity checks will not allow SIP-router to start:

the version of the library the TLS module was compiled with is "too different" from the library used at runtime. The versions should have the same major, minor and fix level (e.g.: 0.9.8a and 0.9.8c are ok, but 0.9.8 and 0.9.9 are not)

the openssl library used at compile time and the one used at runtime have different kerberos options

By default tls_force_run is disabled.

Example 35. Set tls_force_run parameter

...
modparam("tls", "tls_force_run", 11)
...

1.9.27. config (string)

Sets the name of the TLS specific config file.

If set the TLS module will load a special config file, in which
different TLS parameters can be specified on a per role (server or
client) and domain basis (for now only IPs). The corresponding module
parameters will be ignored.

If the file name starts with a '.' the path will be relative to the
working directory (at runtime). If it starts
with a '/' it will be an absolute path and if it starts with anything
else the path will be relative to the main config file directory
(e.g.: for ser -f /etc/ser/ser.cfg it will be relative to /etc/ser/).

By default no config file is specified.

The following parameters can be set in the config file, for each
domain:

tls_method

verify_certificate

require_certificate

private_key

certificate

verify_depth

ca_list

crl

cipher_list

All the parameters that take filenames as values will be resolved
using the same rules as for the tls config filename itself: starting
with a '.' means relative to the working directory, a '/' means an
absolute path and anything else a path relative to the directory of
the current ser main config file.

SIP-router acts as a server when it accepts a connection and as a
client when it initiates a new connection by itself (it connects to
something).

1.11. History

This module was put together by Jan Janak <jan@iptel.org> from code from the experimental tls core addon (http://cvs.berlios.de/cgi-bin/viewcvs.cgi/ser/experimental/tls/), code originally written by Peter Griffiths and later maintained by Cesc Santasusana and from an iptelorg tls code addon, written by Andrei Pelinescu-Onciul <andrei@iptel.org>. Jan also added support for multiple domains, a tls specific config, config reloading and a tls specific select framework.

For ser/sr 3.1 most of the TLS specific code was completely
re-written to add support for asynchrounous TLS and fix several
long standing bugs.